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Astronomy 161
An Introduction to Solar System Astronomy
Prof. Scott Gaudi

Lecture 17: On the Shoulders of Giants:
Isaac Newton and the Laws of Motion

Key Ideas:

1st Law of Motion:
Objects in motion remain in motion unless acted upon by an outside force.

2nd Law of Motion:
Acceleration is proportional to the force & inversely proportional to the mass (F=ma)

3rd Law of Motion:
To every action there is an equal an opposite reaction.

Isaac Newton (1642-1727)

Isaac Newton was born in Woolsthorpe in rural England on Christmas Day of 1642 (Julian, England had not yet adopted the Gregorian Calendar).

Quiet, irascible, and solitary as an adult (he never married). He was always fearful that others would steal from him.

He Graduated from Cambridge in 1665 at the age of 23.


The Plague Years

During the bubonic plague epidemic of 1665-1666, Cambridge closed and Newton went home to Woolsthorpe.

Spent two years in Woolsthorpe, during which

All this in his early 20s. He was publish none of it until many years later.

Lucasian Professor

In 1669, at age 26, he became the Lucasian Professor of Mathematics at Trinity College, Cambridge. [Note: the current holder of the Lucasian Professorship is Stephen Hawking]

Principia Mathematica

In 1684, Newton was prevailed upon by Edmond Halley to publish his work on motion and gravitation.

It took Newton about 3 years to reproduce his earlier work.

Halley paid the publication expenses out of his own pocket, after wheedling, cajoling, and flattering Newton into finishing it.

The results were published by the Royal Society of London in 1687 as the Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy).


Newton's Synthesis

The Principia is one of the most important books in history:

Unified all motions into three simple laws.


First Law of Motion

Every body will stay in a state of rest or uniform motion in a straight line unless that state is changed by forces impressed upon it.
Often called the Law of Inertia.

Speed versus Velocity

All motion is composed of two parts:

The combination is called the VELOCITY:

Change in motion is acceleration:


Second Law of Motion

The size of an acceleration is directly proportional to the force applied, and inversely proportional to the mass of the body. Further, the acceleration will take place in the same direction as the applied force.
Expressed Mathematically:

a = F/m

Alternatively, it can be written as

F = ma

In both cases:
F = force applied to a body
m = mass of the body (e.g., in kilograms)
a = acceleration the body experiences in response to the force applied

Force, Mass, & Acceleration

The second law has two parts:

1) Quantifies the idea of a force in terms of its effects on a massive body.

2) Forces and accelerations have a direction:


Application to Planetary Motion

Planets are continually changing the speed and direction of their motion as they orbit the Sun.

Why does the speed and direction change?

They are thus accelerating in response to an applied force.

What force?

The Force of Gravity.

Third Law of Motion

For every force applied to a body, there is an equal and oppositely directed force exerted in response.
This is commonly re-stated as:
To every action there is an equal & opposite reaction.

Forces come in pairs

The third law brings together the first and second laws, which deal with single bodies.

Unifies them in the case of the interaction between two (or more) bodies via forces.


A Complete Description of Motion

Newton's laws of motion provide a complete, quantitative explanation of the motions of objects.

The Mathematics of Change

The full mathematical statement of the Newton's Laws of Motion required the invention of a new mathematical language: the Calculus

Independently invented by Newton & Leibnitz, Calculus is the mathematics of change:

The Calculus provides a powerful framework within which we can explore the motion of objects, from the fall of an apple, to the orbit of the planets about the Sun.


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